This invention relates to a UV curable adhesive composition, an adhesive obtained on curing the UV curable adhesive composition, and a double-face adhesive tape having a layer of such adhesive.
There has hitherto been employed, as an adhesive layer for a double-face adhesive tape, an adhesive obtained on curing an acrylic adhesive composition. Such acrylic adhesive composition usually contains, as main component, an acrylic acid ester of an alkyl alcohol having 4 to 14 carbon atoms. A cured product of the acrylic acid ester is poor in cohesion, although it has a glass transition temperature of -60.degree. to -40.degree. C. and superior tackiness as an adhesive.
Consequently, several other monomers are contained in the acrylic adhesive composition of improving both adhesion and cohesion in a balanced manner. Examples of these monomers include (i) an ester of lower alcohols and acrylic or methacrylic acid, such as methyl acrylate, ethyl acrylate, propyl acrylate, methyl methacrylate or ethyl methacrylate, (ii) vinyl monomers, such as vinyl acetate or vinylidene chloride, and (iii) monomers containing reactive groups, such as acrylic acid, methacrylic acid, hydroxy ethyl acrylate, hydroxy propyl acrylate, hydroxy ethyl methacrylate, hydroxy propyl methacrylate, glycidyl methacrylate, N-methylol acrylamide and diacetone acrylamide.
Such acrylic adhesive composition, composed of these monomers and the above-mentioned acrylic acid ester of the alkyl alcohol, is prepared into e.g. a coatable adhesive by polymerization by solution polymerization or emulsion polymerization followed by curing on addition of a cross-liking agent.
However, since the solvent or water need to be scattered off during the curing process, a large quantity of the heat energy is required. Besides, it is necessary to take measures against environmental pollution such as by solvent recovery or disposal of exhaust gases.
Consequently, there have been proposed, as a solution-free adhesive which may be produced without employing the solvent, an adhesive which may be produced on radiating ultraviolet rays to an adhesive composition composed of an alkyl alcohol-acrylic acid ester monomer and components copolymerizable therewith, and an adhesive tape prepared from such adhesive (Belgium, 675,420, 5/1966). In such case, acrylic acid esters of lower alcohols or alkyl alcohols having 4 to 14 carbon atoms, such as methyl acrylate or ethyl acrylate, are employed as monomer components curable by UV rays.
However, since methyl acrylate or ethyl acrylate have low boiling point and a strong smell, problems are raised in handling and labor safety if such compounds are used as monomer components of solvent-free adhesive. Besides, the adhesive obtained on curing the composition composed of base monomers by irradiation with UV rays also exhibits a strong disagreeable smell. Meanwhile, among the monomer components of the adhesive in which a solvent is used during curing, methacrylates such as methyl methacrylate, vinyl acetate, as a vinyl monomer, and an acrylic amide monomer, are not polymerized by UV rays, so that these can not be employed as monomer components of the solvent-free adhesive.
On the other hand, in producing a double-face adhesive tape, it is desirable that adhesive strength be improved in such a manner that the phenomenon of destruction of adhesion between the adhesive and an article to be stuck is not the interfacial peeling occurring at an interface therebetween but is the destruction of cohesion in which the adhesive layer of the adhesive tape itself is destroyed. To this end, it is desirable to increase the thickness of the adhesive layer.
It is however difficult to form a thick adhesive layer of an adhesive tape by employing a conventional adhesive which employs a solvent for curing, That is, it is difficult to form a thick adhesive layer by a method consisting in coating an adhesive composition obtained by solution polymerization or emulsion polymerization on a tape-shaped base sheet and vaporizing the solvent or water.
There is also known a method consisting in employing a hot melt adhesive as an adhesive and molding it into a tape by an extruder for forming a thick adhesive layer. However, the adhesive tape produced in this manner is low in thermal resistance and in operability due to sticking when formed into a tape.
These problems may be overcome by employing a solvent-free adhesive obtained on curing with UV rays. However, if the adhesive layer is increased in thickness, the adhesive tends to be stuck to a blade of a metal mold or a creel-shaped mold during pinching. Besides, if the adhesive layer is increased in thickness, the adhesive tends to be exuded when the double-face tape is coiled into a roll, with the exuded adhesive being stuck to itself to render it impossible to rewind and re-use the tape. Besides, there is raised a problem of differential adhesive strength between an irradiated surface and a non-irradiated surface of an adhesive tape being prepared. That is, a transparent polyester film processed by a releasing treatment so as to be peeled off is laminated on the surface of a double-face tape irradiated with the UV rays, while a release paper is provided on the reverse surface of the tape with an adhesive layer in-between, so that the reverse surface is not irradiated with UV Rays and hence the adhesive strength on the reverse surface becomes lower than that on the front surface. If the UV rays are adapted to be radiated onto both the top and bottom surfaces of the adhesive tape in order to cope with the differential adhesive strength, it becomes necessary to provide a complicated equipment.